Defiance Flour Mill, Historic flour mill in Ruthven Street, Toowoomba, Australia.

Defiance Flour Mill is a four-storey brick building at 269–291 Ruthven Street, designed by Toowoomba architect William Hodgen and built in 1911 by WT Smith. A fire damaged it less than two years later, and the rebuilt structure forms the core of the present site. Grain sheds with handling machinery and a capacity of 28,000 bags were added to the north in 1914, and silos built from the 1960s now dominate the site.

The Defiance Milling Company began in 1898 when Patrick O'Brien and Ellen O'Brien partnered with miller George Crisp, operating from Russell Street. In November 1901 the firm bought the former Gisler's mill on Ruthven Street, keeping a Gisler brother as miller-in-charge. A new four-storey brick mill designed by William Hodgen and built by WT Smith rose on the site in 1911, was badly damaged by fire under two years later, and was rebuilt as the core of today's complex.

The mill shaped the local business district and served as a workplace for many Toowoomba families who depended on its operations. People walked past this building daily on their way through the commercial heart of town.

The mill stands at 269–291 Ruthven Street, beside the Toowoomba Foundry and near the Toowoomba Railway Station, so it is easily reached on foot from the station end of the street. It is a heritage-listed industrial site and not a public museum, so the four-storeyed brick mill and the silos that dominate the site are viewed from the street and footpath.

Ellen O'Brien took over the business after her husband's death in 1906 while raising ten children. Her achievement as a mill director during a time when women rarely held such industrial roles is often overlooked.